The UK's Met Office said on X it had received many reports of an "illuminated swirl" in the sky on Monday evening likely caused by a rocket that had blasted off earlier from Cape Canaveral in Florida.
"Don't panic! No, it wasn't a UFO but the degassing of a SpaceX Falcon 9 rocket," said the French state weather forecaster Meteo France in the country's central Loire Valley.
Pictures of the vivid swirl were also posted by social media users in countries including Sweden, Poland, Hungary and Croatia.
People across Europe may have had the chance to see the phenomenon -- as long as it was not obscured by clouds, planetary scientist James O'Donoghue of the UK's University of Reading told AFP.
The spiral was created after the rocket's first-stage booster, which blasted it off the ground, separated and the upper stage took over, he said.
As the booster fell back to Earth, it vented leftover fuel, which froze into reflective crystals.
"These crystals caught the sunlight, creating the bright spiral pattern in the sky," O'Donoghue said.
"The spiral shape happened because the tumbling rocket was spinning as it released the fuel."
- Spy satellite? -
The swirl was created by the launch of SpaceX's NROL-69 mission. Exactly what it was carrying into space was classified.
But the mission was conducted for the National Reconnaissance Office, which manages the US military's spy satellites.
"The Falcon 9 reusable rocket booster returned safely to Landing Zone 1 after delivering the national security payload to orbit," the office said in a statement.
A similar spiral was spotted above New Zealand in 2022, also from the workhorse Falcon 9 rocket, according to reports at the time.
The venting of the gas that creates these spirals is not dangerous, O'Donoghue said. It can happen to other rockets, but the sheer number of Falcon 9 launches makes it a likely suspect.
The timing of this launch -- during the European twilight -- as well as the season, cloud cover and other factors affect whether people on the ground can see such spirals, O'Donoghue said.
Billionaire Elon Musk's SpaceX has taken a "fail fast, learn fast" approach that has helped it become the world's dominant launch services provider.
The company's massive Starship rocket exploded during its latest test flight this month, with social media footage showing red-hot debris raining down over the Bahamas.
Musk's status as one of President Donald Trump's closest advisors, and his influence over federal regulators, have raised concerns about potential conflicts of interest.
Eric Lagadec, an astrophysicist at the Cote d'Azur Observatory in France, reposted a video on the social media platform Bluesky that was apparently taken in Poland of the spiral rising in the sky.
Along with a gif of Musk dancing, Lagadec said: "I have a feeling this is a guy who likes to mess things up on Earth and above!"
dl/rlp/js
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